1. Field of the Invention
This invention relates to an improvement in an electrically heatable transparency such as may be used in a vehicle having deicing, defrosting, or defogging capabilities, and in particular to an improvement in the temperature control of the heatable transparency to protect the transparency against overheating.
2a. Technical Considerations
It has been known to pass electric current through a transparent conductive coating on a transparency in order to raise its temperature. Generally the transparency includes a pair of substantially parallel bus bars on opposite sides of the area of the windshield to be heated. The bus bars have a low resistivity relative to the coating and are connected by leads to a power source to distribute current from the power source through the coating.
One mode of failure of heatable transparencies is overheating of the coating or bus bar. The overheating may result from passing current through a defective or damaged bus bar. Overheating may also result from a discontinuity or gap in the conductive film which may cause arcing. In addition, the relative position and length of the bus bars may effect the flow of the current through the conductive coating and Produce localized areas of elevated temperature, or "hot spots", in the transparency. Hot spots generally occur when the opposing bus bars are not of equal length so that there is increased current flow from the longer bus bar to the shorter bus bar. Automotive windshields are typically trapezoidal in shape and in order to heat the side portions of the windshield, the lower bus bar is extended into these areas. As a result, the length of the lower bus bar is greater than that of the upper bus bar and hot spots are produced near the ends of the upper bus bar. Overheating and arcing can damage the plastic interlayer or in severe cases damage the laminated transparency.
It would be advantageous to have capabilities to monitor the temperature of the heatable transparency so as to prevent overheating that may damage the transparency or render the entire heating system inoperative.
2b. Patents of Interest
U.S. Pat. Nos. 3,789,191 and 3,789,192 to Spindler teach a temperature sensor for use in an electrically heated, laminated window which includes a resistance filament wire wrapped around a core member. The sensor is encapsulated within a casing material which in turn is embedded within the plastic interlayer of the window. The casing material has a heat deflection temperature greater than the plastic interlayer so as to avoid fracture and shortening of the resistance filament against an electrically conductive coating during the high temperature and high pressure conditions of laminating.
U.S. Pat. No. 4,057,671 to Shoop, U.S. Pat. No. 4,323,726 to Criss et al., and U.S. Pat. Nos. 4,543,466 and 4,668,270 to Ramus teach a heated laminated window with an electroconductive coating or a wire grid extending between a pair of generally parallel bus bars. Current passes from one bus bar, through the coating or wire grid, to the opposite bus bar to heat the window.
U.S. Pat. No. 4,565,919 to Bitter et al. teaches a crack detector for an electrically conductive windshield. The circuit includes structure for monitoring the resistance of the conductive film of the windshield and interrupting power applied to the film when the resistance assumes a value indicative of element cracking.
U.S. Pat. No. 4,610,771 to Gillery teaches a transparent, multiple-layer film of metal and metal oxide layers for use as a high transmittance, low emissivity coating.